What happened?

The space shuttle Challenger was ready to launch on the morning of Tuesday, January 28, 1986.

Figure 1. Prepared to launch

This was a symbiolic flight.

Figure 2. The crew

The night before the launch there was a 3 hour teleconference between Morton Thoikol, Marshall Space Flight Center, and Kennedy Space Center. The subject of the discussion was the sensitivity of O-rings, which seal the joints in booster rockets, to low temperatures.

Figure 3. O-rings seal joints on booster rockets

The discussion considered the data below.

Upon inspection of this plot, most of the participants concluded that, since the plot is “U” shaped, there was no evidence that temperature affected O-ring failure.

O-ring failure as a function of temperature

O-ring failure as a function of temperature

In contrast, at least one participant, Roger Boisjoly, stated that “temperature was indeed a discriminator”.

Regardless, the Challenger was launched the following morning.

Figure 4. Challenger launch (NASA)

73 seconds into the flight, the shuttle exploded.

Figure 5. The O-rings failed

…

What should have happened?

Following the accident, President Regan appointed William Rogers to lead a committee to investigate the incident.

The committee concluded that “A combustion gas leak through the right Solid Rocket Motor aft field joint initiated at or shortly after ignition eventually weakened and/or penetrated the External Tank initiating vehicle structural breakup and loss of the Space Shuttle Challenger.”

Further, the committee concluded that “careful analysis of the flight history of the O-ring performance would have revealed the correlation of O-ring damage in low temperature”.

To understand the message from the committee, let’s look at the data:

Flight Temp Pressure O.ring Number
1 66 50 0 6
2 70 50 1 6
3 69 50 0 6
4 68 50 0 6
5 67 50 0 6
6 72 50 0 6
7 73 100 0 6
8 70 100 0 6
9 57 200 1 6
10 63 200 1 6
11 70 200 1 6
12 78 200 0 6
13 67 200 0 6
14 53 200 2 6
15 67 200 0 6
16 75 200 0 6
17 70 200 0 6
18 81 200 0 6
19 76 200 0 6
20 79 200 0 6
21 75 200 2 6
22 76 200 0 6
23 58 200 1 6

The number of O-rings that failed during 23 total flights was recorded witht the temperature and pressure of the joints at launch.

O-ring failure as a function of temperature

O-ring failure as a function of temperature

O-ring failure as a function of temperature

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